Personally I would wait. If you buy now, in a month your computer will be 'outdated'. Lol.
Ok seriously though, technology is always going to be that way with products being replaced with new products. However, it is my opinion that because of the drastic architecture changes, it would be well worth it to get the next gen capabilities. With that said, the new Nehalem processers are said to increase performance by 30% due to the doing away with fsb and replaced with quickpath, new SEE instructions, improved algorithms (branch predictors), and last but also very very important - the reintroduction of hyper-threading. This means that your quad cores would be able to execute 8 threads simultaneously, and on certain versions with two dies in each package actually be able to execute 16 threads simultaneously. There are other improvements that I didn't mention here. My whole point is that its such a huge change and improvement, I would feel like I'm totally wasting money if I bought current technology instead of waiting for a month. 'Current' games might not take advantage of this, but future games will. Why 'outdate' yourself already?
The other reason that I say this is because you won't be able to upgrade your system to the new core i7's in the future either. I might have a different opinion if all your other hardware could stay the same and you could just swap in a core i7 processor later when they become cheaper, but since its such as huge change, not only with your cpu become outdated in a month, but your motherboard and ram will (sort of) too. I say 'sort of' for ram because theres going to be a whole new sleuth of ram coming out since you can't overclock ram past 1.7v with a Nehalem without damaging the cpu. Therefore manufacturers are forced to improve their modules now to match or increase performance over their current line of performance products which usually require 1.8-2.1v.
If you are building a new system, I would recommend waiting until the core i7's are in the market. The only thing I wouldn't go 'overboard' on when you build your new system is video cards. Get yourself a VERY GOOD card cheap, like the Ati 4850 for $150-$200. I say this because when the ATI DX11 cards come out in July `09, thats when you should drop your $$ on the latest there too. Besides, a Nehalem with a 4850 should be able to run every game today very smoothly, including crysis, at max settings without any choppiness on a 1920x1200 resolution.
Hope my opinion helps!